This invention generally relates to welding apparatus and methods and more particularly relates to a system and method for laser welding an inner surface of a small diameter tubular member, which tubular member may be a repair sleeve disposed in a nuclear heat exchanger heat transfer tube.
Although laser welding apparatus and methods are known, it has been observed that these apparatus and methods have a number of operational problems associated with them that make such apparatus and methods less than completely satisfactory for welding an inner surface of a small diameter tubular member. However, before these problems can be appreciated, some background is desirable as to the structure and operation of a typical nuclear heat exchanger.
In a typical nuclear heat exchanger or steam generator, a heated and radioactive primary fluid flows through a plurality of U-shaped tubes, each of the tubes having a fluid inlet and a fluid outlet end. The inlet and outlet ends of the tubes are received through holes in a tubesheet disposed in the heat exchanger for supporting the tubes. The heat exchanger defines an inlet plenum chamber below the tubesheet, which inlet plenum chamber is in communication with the inlet ends of the tubes. The heat exchanger also defines an outlet plenum chamber below the tubesheet and isolated from the inlet plenum chamber, the outlet plenum chamber being in communication with the outlet ends of the tubes. During operation of the heat exchanger, a heated and radioactive primary fluid flows into the inlet plenum chamber and enters the inlet ends of the tubes to flow through the tubes. After flowing through the tubes, the primary fluid then flows through the outlet ends of the tubes and into the outlet plenum chamber. The primary fluid next flows out the outlet plenum chamber to exit the heat exchanger. A nonradioactive secondary fluid having a temperature less than the primary fluid simultaneously surrounds the exterior surfaces of the tubes above the tubesheet as the primary fluid flows through the tubes. Thus, as the heated primary fluid flows through the tubes, it gives-up its heat to the secondary fluid surrounding the exterior surfaces of the tubes to produce steam that is used to generate electricity in a manner well known in the art.
Because the primary fluid is radioactive, the heat exchanger is designed such that the radioactive primary fluid flowing through the tubes does not commingle with and radioactively contaminate the nonradioactive secondary fluid surrounding the exterior surfaces of the tubes. Therefore, the tubes are designed to be leak-tight so that the radioactive primary fluid remains separated from the nonradioactive secondary fluid to avoid commingling the primary fluid with the secondary fluid.
Occasionally, however, the heat exchanger tubes may degrade and thus may not remain leak-tight due, for example, to tube wall intergranular cracking caused by stress and corrosion occurring during operation of the heat exchanger. Therefore, the tubes are inspected to detect such stress corrosion cracking or degradation. If stress corrosion cracking is detected at a particular location in the wall of the tube, then the tube is "sleeved" at that location. When sleeving is performed, a tubular metal sleeve is inserted into the tube, so as to cover the degraded portion of the tube, and affixed thereto typically by expanding the sleeve into intimate engagement with the tube. In this manner, the sleeved tube remains in service although degraded.
However, the elastic properties of the metal sleeve may cause the sleeve to experience partial "spring back" after expansion. This phenomenon of "spring back" may in turn cause a relatively small gap to exist at the sleeve-to-tube interface. Such a gap is undesirable because the gap defines a flow path between the sleeve and the tube, which flow path may allow the radioactive primary fluid to flow through any crack in the tube and thereafter undesirably commingle with the nonradioactive secondary fluid. Therefore, it is desirable to fuse the sleeve to the tube by forming, for example, two spaced-apart weldments circumscribing the inner surface of the sleeve in order to seal the gap and the flow path defined thereby. In this regard, laser welding has been used to fuse such a sleeve to the tube.
Laser welding devices are known. A system and method for laser welding a tube is disclosed in U.S. Pat. No. 5,182,429 titled "System And Method For Laser Welding The Inner Surface Of A Tube" issued Jan. 26, 1993 in the name of William E. Pirl, et al. and assigned to the assignee of the present invention. This patent discloses a system and method for laser welding a sleeve to the inner surface of a heat exchanger tube in order to repair the tube. The Pirl, et al. system comprises an elongated tubular housing having a rotatable distal portion connected to a non-rotatable proximal portion, a fiber-optic cable for conducting remotely generated laser light into the tubular housing, a beam deflection mechanism supported within the distal portion of the housing and a reflector for radially directing and focusing laser light received from the fiber-optic cable toward the inner wall of the sleeve to weld the sleeve. In order to weld around the inner wall of the sleeve, the Pirl, et al. device provides an electric motor within the proximal portion of the tubular housing to rotate the distal portion of the housing and the reflector supported therein.
However, applicant has observed that such a prior art laser welding device and its associated motor are unsuitable for welding a sleeve having a relatively small inner diameter (e.g., an inner diameter equal to or less than approximately 0.313 inch). This is so because the size of commercially available motors is necessarily larger than the relatively small inner diameter of the sleeve. Therefore, a problem in the art is to rotate the distal portion of the tubular housing and the reflector supported therein without locating the motor within the proximal portion of the housing, so that relatively small diameter sleeves can be welded.
Therefore, what is needed are a system and method for laser welding an inner surface of a small diameter tubular member, which tubular member may be a repair sleeve disposed in a nuclear heat exchanger heat transfer tube.